The present invention relates to a method and apparatus for treating water using ozone generated with an electrolytic ozonizer.
Ozone is drawing researchers' attention as a powerful and clean oxidizer, and ozone is increasingly used as a water treating agent. Among the many advantages of ozone for use in water treatment, the following are notable: since it decomposes to oxygen, ozone does not remain in the treated water unlike conventionally used chlorine-based chemicals,. and ozone decomposes so rapidly that no residual toxicity which may lead to secondary pollution occurs.
Ozone for as an oxidizer is conventionally generated by two basic processes, namely, electric discharge and electrolysis. In the first method, an electric discharge such as a silent discharge is produced in bone dry air or oxygen gas to generate ozone. The ozone generated by electric discharge is characterized in that its concentration is relatively low being less than or equal to about 10%, the gaseous product contains substantially no water and is contaminated by the discharge electrode material. If air is used as an ozone source, the nitrogen component is oxidized to generate NOx, and a significant amount of NOx is present in the product gas.
When the ozone generated by electric discharge is used in water treatment, various disadvantages arise. In particular, because of its low concentration, the ozone dissolves in the water to insufficiently treat the water resulting in low operational efficiency. Additionally, dry ozone takes a longer time to dissolve in the water to be treated than wet ozone. Furthermore, the generated ozone contains the electrode material as an impurity which is not a problem as long as the contaminated ozone is used in the treatment of ordinary water, but which must be removed when pure water, in particular ultrapure water, is to be produced.
To avoid the above-noted disadvantages of ozone generation by electric discharge, the industry is shifting attention to "electrolytic ozone" generated by the electrolysis of water as described in U.S. Pat. No. 4,416,747 and J. Elec. Chem. Soc., 132, p. 367 ff (1985), although electrolysis is somewhat lower in power efficiency as compared to electric discharge. In this approach, water is electrolyzed using lead dioxide, gold, platinum, etc. either as electrodes alone or as electrode substances attached to opposite sides of a solid polymer electrolyte (SPE) diaphragm to provide a "SPE type" electrode structure. Ozone, the product of water electrolysis, is evolved at the anode as a mixture with oxygen. The product ozone or "electrolytic ozone" generated in the liquid system is saturated with water vapor, and dissolves readily in the water to be treated. Furthermore, the ozone can be obtained in concentrations higher than 15 wt. %. When electrolytic ozone is to be employed in water treatment, the conventional practice comprises either supplying part of the water to be treated directly into the anode compartment such that the ozone in the anolyte acts directly on the water, or injecting the ozone-containing anolyte directly into the water to be treated. The first method has the advantage of fully exploiting the above-described features of electrolytic ozone. On the other hand, the presence of the water to be treated in the electrolytic cell causes contamination of the anolyte by impurities in the water, resulting in the generation of low-purity ozone. Furthermore, such impurities are deposited onto the electrodes and the diaphragm in the cell thereby reducing the efficiency of electrolysis. Moreover, the anolyte may backflow as a result of variations in the supply pressure of the water to be treated.
The above-noted problems are absent from the second approach wherein ozone-containing anolyte is injected into the water to be treated. However, the electrode substances are present in trace amounts in the anolyte, and thereby contaminates the water being treated. Because of this disadvantage, the second approach is unsuitable for use in the treatment of high-purity water, i.e., ultrapure water, for use in the semiconductor industry.